"Outlook data file not closed properly" and takes a long time to c

Hello, I got a user with an exceptional big mailbox (around 2GB) running
Outlook 2003 in cached mode and he complains that he often gets the error
message "The mailbox datafile was not closed properly and must be checked for
errors" which takes quite a long time due to sheer size of the file.
The user is using a laptop with a dockstation and complains that the problem
often occurs when he undocks the computer so do you have any ideas what might
cause the problem with the mailbox datafile and how to prevent it in the
future?